A typical prior art plate-shaped light-emitting device of the kind defined above is called a light guide plate, and has an edge-mounted light source and a rear surface that is provided with a light-refracting and light-reflecting structure, which redirects the light out of the front surface of the light guide plate. An example of such a light guide plate is disclosed in US-2010/0245718. A light source is arranged at an edge surface of the light guide. Due to total internal reflection, the light propagates with a main direction of propagation in parallel with the front and rear surfaces, which are the major surfaces of the light guide. However, the rear surface of the light guide is provided with a three-dimensional structure, and more particularly wedge shaped portions, which inhibits the total internal reflection and refracts the light out of the front surface of the light guide plate. A reflecting layer arranged on the rear surface of the light guide assures that any rearly refracted light is reflected out through the front surface of the light guide plate. In this prior art example, the light guide plate is used as a backlight in an LCD TV screen. That is a typical use of light guide plates, and another example is a mobile telephone display, and more general any type of LCD screen, large or small. The three-dimensional structure can also be provided at the front surface, while still refracting light out of the front surface. Examples of three-dimensional structures are knobs, pits, or dots of white paint. Another kind of application is in aesthetic applications. In the latter case, the light guide plates are often used in common arranged adjacent to each other in order to form one larger light-emitting surface.
Over the years large efforts have been invested in trying to couple out as much light as possible through the front surface and minimising the loss of light in other directions.